Control system using an electric motor

ABSTRACT

The electric motor drive includes an electric motor ( 10 ); an end stage ( 12 ) for triggering the electric motor; a trigger circuit ( 18 ) for pulse-width-modulated triggering of the end stage, which is associated with the end stage ( 12 ) and which includes an overload stage ( 20 ) for detecting an overload on the electrical motor according to a temperature of the electric motor and a device for obtaining a temperature from a pulse-width-modulation signal ( 16 ) generated by the trigger circuit ( 18 ). A preferred embodiment includes a device for integrating a differential power (P diff ) equal to an instantaneous power (P mom ) minus an equipment specific limit power (P limit ) in predetermined time intervals to obtain an integrated power (P new ). The motor is turned on or off by comparison of the integrated power with various limiting values.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electric motor drive, having an electricmotor that is triggerable via an end stage.

2. Prior Art

Electric motor drives of this generic type are known. They are used forinstance in motor vehicles, as control motors. They are then operated ata supply voltage furnished by a motor vehicle battery. The electricmotors embodied as direct current motors can be used in thermallycritical areas of the motor vehicle, such as in the immediate vicinityof an internal combustion engine, among other areas. It is also known toprovide electric motor drives with an overload protection that isintended to protect the electric motors against irreparable thermaldamage, for instance if sluggishness suddenly occurs.

To protect electric motors against thermal overload, it is known todetect a housing temperature and/or an armature winding temperature ofthe direct current motor via a temperature sensor or a bimetallicelement. The electric motor is rendered currentless if a permissibletemperature is exceeded.

It is also known for electric consumers, such as motor end stages fordirect current motors in motor vehicles, to be triggered with apulse-width modulation signal. The electric consumer is connected to avoltage source or disconnected from the voltage source in accordancewith a duty factor of the pulse-width modulation signal.

From International Patent Disclosure WO 94/27349, an electric motordrive is known in which a motor end stage is connected to a triggercircuit, and the trigger circuit generates an overload signal as afunction of a temperature of the electric motor. A temperature of theelectric motor can be calculated here from a power loss, or a variableproportional to it, on the basis of measured motor data during the dutycycle of the electric motor and integrated; for forwarding the overloadsignal, the integration value is compared with a predeterminablethreshold value. A disadvantage here is that the requisite measurementof motor data during the duty cycle of the electric motor dictateseffort and expense for measurement that moreover involves error, so thatan exact overload signal cannot be ascertained.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved electricmotor drive, which is simpler and less complex than those of the priorart.

According to the invention the electric motor drive comprises anelectric motor; an end stage comprising means for triggering theelectric motor; a trigger circuit associated with the end stage, thetrigger circuit having a microprocessor and comprising means forpulse-width-modulated triggering of the end stage, the trigger circuitincluding an overload stage, the overload stage comprising means fordetecting an overload on the electrical motor according to a temperatureof the electric motor and means for obtaining the temperature from apulse-width-modulation signal generated by the trigger circuit withoutmeans for directly measuring operating parameters of the electric motor,the means for directly measuring operating parameters including currentmeasuring means or temperature measuring means.

The electric motor drive of the invention has the advantage over theprior art that in a simple way, an exact overload signal for turning offthe electric motor drive can be furnished. Because the temperature ofthe electric motor is obtained from a pulse-width modulation signal ofthe trigger circuit, measurement of motor data during electric motoroperation can be dispensed with. The complexity of the circuitry is thussimplified considerably. Sources of error associated with measuringmotor data during operation are also circumvented, so that the signalcan be generated with high accuracy. Ascertaining the motor temperatureis done solely on the basis of already existing signals, namely thepulse-width modulation signal for triggering the electric motor. Noother additional direct measurements of operating parameters of themotor, such as a current or a temperature, are necessary.

Other preferred features of the invention will become apparent from theother characteristics recited in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in detail below in terms of an exemplaryembodiment in conjunction with the associated drawing, which shows ablock circuit diagram of an electric motor drive.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

FIG. 1, in a block circuit diagram, shows an electric motor drive. Theelectric motor drive includes a direct current motor 10, which acts forinstance as a control motor in a motor vehicle. The direct current motor10 is connected to an end stage 12, which has switch means forconnecting the direct current motor 10 to an energy source 14. In motorvehicles, the energy source is a motor vehicle battery, for example. Theswitching stage 12 is supplied with a control signal 16, which isfurnished by a trigger circuit 18. By means of the trigger circuit 18, apulse-width-modulated triggering of the switching stage 12 is effected,so that a clocked mode of operation of the direct current motor 10 is

During the intended use of the direct current motor 10, this motor, forinstance from sluggishness or the presence of an obstacle in acontrolled path of a final control element that is movable with thedirect current motor 10, can become mechanically and thermallyoverloaded, so that temperature monitoring of the direct current motor10 is necessary in order that an overload signal will be generated atthe correct time. To that end, an overload stage 20 is integrated withthe trigger circuit 18.

The ascertainment of a temperature of the direct current motor 10 bymeans of the overload stage 20 is done on the basis of a temperaturemodel, as follows:

It is known that the temperature of the direct current motor 10 dependson a delivered and an output power. The equation is

P _(t) =P _(del) −P _(out′)

where P_(t) stands for the temperature power of the direct currentmotor, P_(del) stands for the delivered power and P_(out) stands for theoutput power. The output power P_(out) can be ascertained from anendurance run test, for instance, and assumed to be constant, for apredetermined, known design of the direct current motor.

The delivered power is obtained from the equation

P _(del) =U·I=U ² /R;

U stands for the pulse-width modulated voltage signal 16 specified viathe trigger circuit 18; I stands for the motor current; and R stands forthe armature resistance of the motor. The resistance R can be assumed tobe constant, ignoring any temperature dependency. Experience shows thatthe armature resistance R of the direct current motor 10 fluctuates byapproximately 50% over a temperature range from −40° C. to +85° C., forexample.

To ascertain the temperature of the direct current motor 10, adistinction must be made between two operating situations. In a firstoperating situation (the diabatic case), an armature of the directcurrent motor 10 is in motion, so that via carbon brushes, heat can bedissipated over the entire circumference of a commutator of thearmature. A second situation (the adiabatic case) exists when thearmature of the direct current motor 10 is stopped or is rotating onlyslightly. Then heat dissipation via the carbon brushes from thecommutator is sharply restricted, so that faster heating of the armatureand carbon brush can take place.

Ascertaining a temperature of the direct current motor 10 from thepulse-width modulation signal 16 of the trigger circuit 18 will now beexplained. The temperature is ascertained here from a model calculation;that is, without direct temperature measurement, such as via atemperature sensor or the like. The temperature can be ascertained atcertain time intervals. For instance, as the time interval forcalculating the current temperature power P_(t), the time-slot patternof the digital pulse-width modulation signals can be selected. Forexample, the calculation can be done at each edge at a transition of thedigital signal from OFF to ON and/or from ON to OFF. However, it isexpedient to perform the calculation in a time interval that takes onlyevery n^(th) control value (each n^(th) edge) into account. By means ofa filter, the n^(th) value to be taken into account can be ascertained.

An instantaneous power P_(mom) is obtained from squaring theinstantaneous digital pulse-width modulation signal 16 PWM_(mom). Thiscorresponds to the current output of the pulse-width modulation signal16 to the switching stage 12. That is:

P _(mom) =PWM _(mom) ·PWM _(mom).

A countervoltage occurring in the direct current motor 10 as the rpmincreases is not taken into account but instead is assumed on average tobe a proportional factor that can be taken into account by ensuingparametrizing.

Next, a maximum allowable power P_(limit) that is dependent on thespecific direct current motor 10 is subtracted from the calculated powerP_(mom), resulting in a differential power P_(diff) as follows:

P _(diff) =P _(mom) −P _(limit).

For each time interval (n^(th) edge of the signal 16 ) used forcalculating the instantaneous power, the differential power P_(diff) isintegrated; the equation is:

P _(new) =P _(old) +P _(diff).

If the thus-integrated power P_(new) attains a warning power P_(warn)that can be defined for the direct current motor, then via the triggercircuit 18, the pulse-width modulation signal 16 for the switching stage12 can be varied in such a way that a less power-intensive triggering isdone. This reduces the temperature load on the direct current motor 10.By further integration of the differential power P_(diff′) a definableturnoff threshold P_(max) is attained, at which the direct current motor10 is turned off. At the same time, as the calculation continues in eachdefined time interval, a lowering of the integration value P_(new) isdone linearly by the limit power P_(limit). The direct current motor 10can be turned on whenever the integration value P_(new) has attained adefinable limit P_(min). When the direct current motor 10 is turned onvia the pulse-width modulation signal 16, the integration to the powerP_(new) starts over again, in the sequence described.

By means of a characteristic continuous-operation program that isadapted to the specific direct current motor 10 and its specific usagecondition, the maximum allowable continuous load can be simulated. Theworse possible conditions, for instance with regard to an ambienttemperature and humidity should be taken into account optionally takinginto account forced cooling in conjunction with a representative oraverage expected load moment. The maximum allowable continuous load canbe ascertained by setting the ambient temperature and optionally thehumidity to the worst possible values to be expected. Next, a continuousoperation cycle and/or a load moment is increased in increments, until atemperature of the direct current motor 10 that is still just barelypermissible is reached. On the basis of existing time constants, a valuethat is just below the ascertained allowable value must be ascertained,so as to ascertain the allowable continuous load. Under thesethen-selected load parameters, the direct current motor 10 is operatedin the continuous load mode; stable conditions ensue after a timeequivalent to approximately three to five times a thermal system timeconstant of the direct current motor 10. The pulse-width modulationsignals 16 corresponding to these conditions are detected by the triggercircuit 18 and stored in memory and used in the later temperaturedetermination under usage conditions.

The limit power P_(limit) is ascertained from the increase over time inthe integration value P_(new), which is initially still calculatedassuming P_(limit)=0 and thus rises monotonously. Taking thethen-ascertained value for the limit power P_(limit) has the effect thatat the maximum allowable load on the direct current motor 10, theintegration value P_(new) remains unchanged. The warning power P_(warn)is defined below the turnoff power P_(max).

To enable taking the aforementioned adiabatic case into account as well,in which extreme heating exists because of the lack of heat dissipationvia the carbon brushes, it is provided that the instantaneous powerP_(mom) filtered with a filter power P_(filter). The filter power isobtained here from the following equation${P_{filter} = {{K \cdot \frac{T_{a}}{\left( {T_{a} + T} \right) \cdot P_{mom}}} + \frac{T}{\left( {T_{a} + T} \right) \cdot P_{filter}}}},$

in which K is an amplification factor, T_(a) is a sampling time, and Tis a tau value.

If the filter power P_(filter) exceeds a warning power P_(warn′), then aswitchover can be made to a less thermally burdensome control strategyfor triggering the direct current motor 10. If the turnoff powerP_(max′) is exceeded, the direct current motor 10 is turned off. Thefilter power P_(filter) then drops with a time constant until a minimalpower P_(min′) is reached. At that time, the filter power P_(filter)remains constant, and the direct current motor 10 can be turned onagain.

The time constant of the filter is on the order of magnitude of thatwith which the direct current motor 10 is heated, by a maximum possibleload, from its operating temperature to a critical temperature.Parametrization is done by analysis of a brief maximum allowable loadwith the critical temperature. This brief load has an order of magnitudeof approximately three times the value of the time constant. Forexample, the maximum allowable load can be caused by a runup of thedirect current motor at maximal load at the least favorable ambienttemperature to be expected. The filter power P_(filter) is initiallyimplemented without the turnoff power P_(max′), and thus the maximumfilter power P_(filter) attained at the maximum load is ascertained. Theturnoff power P_(max′), is then selected to be slightly greater thanthis attained maximum filter power P_(filter). The warning powerP_(warn′) is again selected to be below the turnoff power P_(max)′.

The two aforementioned operating cases, that is, the diabatic and theadiabatic cases, will occur in mixed form when the direct current motor10 is used as intended. In this sense, it is necessary to link theseascertained values. An OR linkage of the warning power P_(warn) in thediabatic case is linked with the warning power P_(warn′) of theadiabatic case. Once again, a turnoff of the direct current motor 10 isdone by means of an OR linkage of the two turnoff powers P_(max) andP_(warn′) of the diabatic and the adiabatic case, respectively. Turningthe direct current motor 10 back on again is possible only if in bothcases turning it back on again is permitted; that is, if the minimalpower P_(min) of the diabatic case and of the adiabatic case are linkedby an AND linkage.

In summary, by a simple design and embodiment of the trigger circuit 18with the overload stage 20, it is possible to vary the pulse-widthmodulation signal 16 for triggering the direct current motor 10. Noadditional temperature measurements or current measurements arenecessary. The overload stage 20 need merely be informed of an initialtemperature. This may for instance be the synthetic specification of aninitial value that corresponds to the worst possible operatingconditions of the direct current motor 10. It is also conceivable for anambient temperature, furnished by other sensors, such as temperaturesensors disposed inside a motor vehicle, to be linked with an OFF timeof the direct current motor 10, in order in this way to generate aninitial value.

What is claimed is:
 1. An electric motor drive, comprising an electricmotor (10); an end stage (12) comprising means for triggering theelectric motor; a trigger circuit (18) associated with the end stage(12), said trigger circuit (18) having a microprocessor and comprisingmeans for pulse-width-modulated triggering of the end stage, saidtrigger circuit (18) including an overload stage (20), said overloadstage comprising means for detecting an overload on the electrical motoraccording to a temperature of the electric motor; means for obtainingsaid temperature from a pulse-width-modulation signal (16) generated bythe trigger circuit (18); means for calculating an instantaneous power(P_(mom)) from the pulse-width-modulation signal (16) and means forintegrating a differential power (P_(diff)) in each of a number ofpredetermined time intervals to obtain an integrated power (P_(new)),said differential power being equal to said instantaneous power(P_(mom)) minus an equipment specific limit power (P_(limit)).
 2. Theelectric motor drive as defined in claim 1, further comprising means forevaluating the pulse-width-modulation signal at each n^(th) edge.
 3. Theelectric motor drive as defined in claim 1, further comprising means forvarying the pulse-width-modulation signal (16) for triggering theelectric motor (10) when the integrated power (P_(new)) reaches anequipment-specific warning power level (P_(warn)).
 4. The electric motordrive as defined in claim 1, further comprising means for turning offthe electrical motor (10), when the integrated power (P_(new)) reachesan equipment-specific turnoff power (P_(max)).
 5. The electric motordrive as defined in claim 4, further comprising means for turning theelectric motor (10) is turned back on again if the integrated power(P_(new)) in one of the predetermined time intervals attains or fallsbelow an equipment-specific minimum power (P_(min)).
 6. The electricmotor drive as defined in claim 1, further comprising means forfiltering the instantaneous power (P_(mom)) obtained from thepulse-width modulation signal (16) with a filter power (P_(Filter)). 7.The electric motor drive as defined in claim of claim 6, furthercomprising means for varying the pulse-width modulation signal (16) fortriggering the electric motor (10) when the filter power (P_(Filter))attains another equipment-specific warning power level (P_(warn′)). 8.The electric motor drive as defined in claim 6, further comprising meansfor turning off the electric motor (10) when the filter power(P_(Filter)) reaches an equipment-specific turnoff power.
 9. Theelectric motor drive as defined in claim 8, further comprising means forturning the electric motor (10) back on again if the filter power(P_(Filter)) has dropped to an equipment-specific minimum power(P_(min)).
 10. The electric motor drive as defined in claim 9, whereinthe filter power (P_(Filter)) decreases with an equipment-specific timeconstant.
 11. The electric motor drive as defined in claim 1, furthercomprising means for varying the pulse-width modulation signal (16) if afirst warning power (P_(warn)) or a second warning power (P_(warn′)) isreached.
 12. The electric motor drive as defined in claim 11, furthercomprising means for turning off the electric motor (10) when a firstturnoff power (P_(max)) or a second turnoff power (P_(max′)) is reached.13. The electric motor drive as defined in claim 12, further comprisingturning the electric motor (10) back on again if a first minimum power(P_(min)) or a second minimum power (P_(min′)) is reached.
 14. Anelectric motor drive, comprising an electric motor (10); an end stage(12) comprising means for triggering the electric motor; a triggercircuit (18) associated with the end stage (12), said trigger circuit(18) having a microprocessor and comprising means forpulse-width-modulated triggering of the end stage, said trigger circuit(18) including an overload stage (20), said overload stage comprisingmeans for detecting an overload on the electrical motor according to atemperature of the electric motor; means for obtaining said temperaturefrom a pulse-width-modulation signal (16) generated by the triggercircuit (18); means for obtaining an instantaneous power (P_(mom)) fromthe pulse-width-modulation signal (16) and means for filtering theinstantaneous power (P_(mom)) obtained from the pulse-width-modulationsignal (16) with a filter power (P_(Filter)).
 15. The electric motordrive as defined in claim 14, further comprising means for evaluatingthe pulse-width-modulation signal at each n^(th) edge.
 16. The electricmotor drive as defined in claim 14, further comprising means for turningoff the electrical motor (10), when an integrated power (P_(new))reaches an equipment-specific turnoff power (P_(max)).
 17. The electricmotor drive as defined in claim 14, further comprising means for turningoff the electric motor (10) when the filter power (P_(Filter)) reachesan equipment-specific turnoff power.
 18. The electric motor drive asdefined in claim 16, further comprising means for turning the electricmotor (10) back on again if the filter power (P_(Filter)) has dropped toan equipment-specific minimum power (P_(min)).
 19. The electric motordrive as defined in claim 17, wherein the filter power (P_(Filter))decreases with an equipment-specific time constant.
 20. An electricmotor drive, comprising an electric motor (10); an end stage (12)comprising means for triggering the electric motor; a trigger circuit(18) associated with the end stage (12), said trigger circuit (18)having a microprocessor and comprising means for pulse-width-modulatedtriggering of the end stage, said trigger circuit (18) including anoverload stage (20), said overload stage comprising means for detectingan overload on the electrical motor according to a temperature of theelectric motor and means for obtaining said temperature from apulse-width-modulation signal (16) generated by the trigger circuit (18)without means for directly measuring operating parameters of theelectric motor, said means for directly measuring operating parametersincluding current measuring means or temperature measuring means. 21.The electric motor drive as defined in claim 20, wherein the pulse-widthmodulation signal (16) is evaluated in pre-selected time intervals. 22.The electric motor drive as defined in claim 20, further comprisingmeans for integrating a differential power (P_(diff)) in each of anumber of predetermined time intervals to obtain an integrated power(P_(new)), said differential power being equal to an instantaneous power(P_(mom)) minus an equipment specific limit power (P_(limit)), saidinstantaneous power being obtained from the pulse-width-modulationsignal (16).
 23. The electric motor drive as defined in claim 22,further comprising means for varying the pulse-width-modulation signal(16) for triggering the electric motor (10) when the integrated power(P_(new)) reaches an equipment-specific warning power level (P_(warn)).24. The electric motor drive as defined in claim 22, further comprisingmeans for turning off the electrical motor (10), when the integratedpower (P_(new)) reaches an equipment-specific turnoff power (P_(max)).25. The electric motor drive as defined in claim 24, further comprisingmeans for turning the electric motor (10) is turned back on again if theintegrated power (P_(new)) in one of the predetermined time intervalsattains or falls below an equipment-specific minimum power (P_(min)).26. The electric motor drive as defined in claim 22, further comprisingmeans for filtering the instantaneous power (P_(mom)) obtained from thepulse-width modulation signal (16) with a filter power (P_(Filter)). 27.The electric motor drive as defined in claim of claim 26, furthercomprising means for varying the pulse-width modulation signal (16) fortriggering the electric motor (10) when the filter power (P_(Filter))attains another equipment-specific warning power level (P_(warn′)). 28.The electric motor drive as defined in claim 26, further comprisingmeans for turning off the electric motor (10) when the filter power(P_(Filter)) reaches an equipment-specific turnoff power.
 29. Theelectric motor drive as defined in claim 28, further comprising meansfor turning the electric motor (10) back on again if the filter power(P_(Filter)) has dropped to an equipment-specific minimum power(P_(min)).
 30. The electric motor drive as defined in claim 29, whereinthe filter power (P_(Filter)) decreases with an equipment-specific timeconstant.
 31. The electric motor drive as defined in claim 22, furthercomprising means for varying the pulse-width modulation signal (16) if afirst warning power (P_(warn)) or a second warning power (P_(warn′)) isreached.
 32. The electric motor drive as defined in claim 31, furthercomprising means for turning off the electric motor (10) when a firstturnoff power (P_(max)) or a second turnoff power (P_(max′)) is reached.